The synthesis of systematically related thio and seleno sugar esters of diorganylphosphinous, -arsinous, and -antimonous acids will be carried out. Compounds having these structures have been found to be reversible inhibitors of muscle phosphorylase, irreversible inhibitors of the glucosidase function of the muscle glycogen debranching enzyme, and to possess antitumor activity. It is proposed that a systematic relationship between the chemical structures of this group of sugar derivatives and their effects on the above biological activities be sought. This will be done by relating the results of enzyme inhibition studies and tumor screening tests to the chemical structures.